Projection system for charged particles

ABSTRACT

A charged particle, in particular ion projector system, has a mask arranged in the path of the charged particle beam and provided with transparent spots, in particular openings, arranged asymmetrically to the optical axis, which are reproduced on a wafer by means of lenses arranged in the path of the charged particle beam. The charged particle beam has at least one cross-over (crosses the optical axis at least once) between the mask and the wafer. Charged particles with an opposite charge to the charge of the reproduction particles are supplied into the path of the reproduction charged particle beam in a defined area located between the mask and the wafer. The limits that define said area are selected in such a way that the absolute value of the integral effect of the space charge on the particles that reproduce the mask structures is as high upstream of said area (seen in the direction of radiation) as the absolute value of the integral effect of the space charge downstream of said area.

FIELD OF THE INVENTION

The invention relates to a projection system for charged particles, inparticular ions, having a mask disposed in the beam path of chargedparticles which mask comprises transparent portions, more specificallyholes, which are disposed in a particular asymmetrical manner withrespect to the optical axis and which are imaged onto a wafer usinglenses disposed in the beam path of the charged particles, wherein thebeam of the charged particles comprises between the mask and the waferat least one cross-over (crosses the optical axis).

BACKGROUND INFORMATION

In the case of the projection systems of this type, problems occur owingto the fact that in the case of identical current density at the maskprovided with the transparent portions, depending upon the mask andarrangement and also the size of the transparent portions, differentionic currents and current density distributions occur behind the mask.The so-called space-charge effect, which occurs owing to the mutualinfluencing of the charged particles once they have passed through thetransparent portions of the mask, causes the charged particles to bedeflected differently, thus causing the charged particles to impact onthe wafer at impact locations (actual impact locations) which differfrom the desired impact location which can produce scale changes andimage distortions.

These problems are particularly serious if the transparent portions inthe mask are disposed in an extreme asymmetric manner with respect tothe optical axis. In order to solve the ensuing problems it has alreadybeen proposed (image Projection Ion-Beam Lithography by Paul A. Millerin SPIE Vol, 1089 Electron-Beam, X-ray, and Ion-Beam TechnologySubmicrometer Lithographies VII I (1989), pages 26 and following) toattach compensating holes (or rather transparent regions) in the mask inorder to achieve an arrangement of transparent regions (holes) whichcreate an arrangement of transparent regions (holes) which aresymmetrical with respect to the ionic-optical axis.

A disadvantage of these aforementioned features resides in the fact thatthe total current increases and thus the effect of the so-calledstochastic space charge, which is based on the direct Coulombianinteraction of the randomly distributed ions, is increased, resulting inthe maximum achievable resolution being impaired. The stochastic spacecharge is according to A. Weidenhausen, R. Spehr, H. Rose, Optics 69,Page 126-134, 1985 proportional to the quadratic root of the totalcurrent.

Apart from this disadvantage, it is also difficult to calculate the sizeand arrangement of the transparent regions which are arranged on themask and assist in the compensating process.

SUMMARY OF THE INVENTION

In order to provide a solution to the problem mentioned in theintroduction and to avoid problems which occur in the arrangement of thetransparent regions which assist in the compensating process, theinvention proposes in the case of a projection system for chargedparticles to provide between the mask and the wafer a dispenser ofparticles which are oppositely charged with respect to the particlesbeing imaged, e.g. a thermionic cathode, for projection systems whichoperate with positively charged ions, for the purpose of directingoppositely charged particles into the beam path of the chargedparticles. Apart from positive ions, the projection system embodying theinvention can also be used to project electrons or to project negativeions, more specifically negative hydrogen ions. These negative ionsrender possible a more convenient mass separation.

The electrically charged particles which are to be directed between themask and the wafer are oppositely charged with respect to the particlesof the main beam. If the imaging particles are positively charged ions,then electrons are used as oppositely charged particles. If the imagingparticles are negatively charged ions, more specifically negativeH-ions, then the oppositely charged particles are positively chargedions. If electrons are used as the imaging particles, then theoppositely charged particles are positively charged ions. The particlescharged oppositely to the charge of the particles of the main stream aredirected in a region whose boundaries are defined by the requirementthat the absolute amount of the integral effect of the space charge onthe particles imaging the mask structures, as seen in the beamdirection, before this region is identical in size to the absoluteamount of the integral effect of the space charge after this region.

For simplification purposes, it is assumed hereinunder that the imagingparticles (main stream) are positively charged ions and that electronsare used as particles for the space charge compensation.

It is not necessary for projection systems comprising a cross-overbetween mask and wafer to direct electrons in the entire region betweenthe mask and wafer since the effect before and after the cross-over ispartially compensated. Since the space charge effect is greater, thecloser the ions are to each other, it is most favourable to direct theelectrons in the region around the cross-over.

In a further embodiment of the invention it is possible for the purposeof defining the region in which the neutralising particles are directedto provide on each side of the region, preferably on both sides of thecross-over, at least one diaphragm which is connected to a voltagesource and whose hole allows the passage of any beam of rays whichcontains the image information originating from the transparent portionsof the mask. Between the diaphragms there is, necessitated by thepotentials which impinge on the diaphragm, a cage for the electrons,which cage substantially prevents the electrons from escaping outwardsso that they can be utilised to their full effectiveness.

A series of diaphragms can be disposed in each case on both sides of thecross-over. The potential on these diaphragms can be changed, wherebythe defined region for the supply of the charged particles for thecompensation of the space charge can be varied. The holes of thediaphragms can be different, in such a manner that the diaphragmsclosest to the cross-over comprise a smaller hole than those furtheraway from the cross-over. The size of the diaphragm holes is however, asalready mentioned, to be tailored to suit the cross-section of the beamof rays which contains the image information originating from the maskin order to transfer the full contents of the image information onto thewafer.

The electron dispenser can be e.g. a LaB6 cathode, for example anindirectly heated LaB6 cylinder which encompasses the ion beam. Or lowenergy secondary electrons are generated with the aid of an electroncanon which emits in particular 1 keV electrons through an hole onto theinner wall of a metallic, especially cylindrical screen whichencompasses the beam and which secondary electrons neutralise the ionbeam.

As already mentioned, an electrically charged particle (e.g. an ion),which passes through a mask hole, is subjected to the influence of afield which is induced by the other ions which move in the directiontowards the wafer plane in the particle-optical column. In the case of aspace charge which is initially assumed to be radially symmetrical theadditional field produced by the space charge has an overall purelyradial effect. A charged particle which without the space charge effectwould arrive at the wafer in the distance R₁ Azimuth φ will thereforewhen taking into consideration the space charge arrive at the wafer atthe identical Azimuth φ but in a predetermined distance ΔR from thedesired impact location.

The deviation ΔR can be expressed mathematically as follows: ##EQU1##

In the equation (1) Z_(w) represents the coordinate of the wafer plane,Z_(m) the coordinate of the mask plane, α represents the respectiveangle of the particle path with the optical axis in the z-direction(propagation of the beam), dR/dα represent the change of the beamposition in the wafer plane owing to the angle change at the location z,dα/dz represent the change in beam direction along the particle-opticalaxis (z-direction) caused by the space charge.

Assuming that the ratio of the radial distances of the respective twoion paths along the optical axis remains constant and that the speedcomponent in the z-direction V_(z) (Z) is identical for all ions, thattherefore--in other words--the effective charge quantity for eachparticle along its path is always the same size, then the followinganalytic expression is produced for the angle change dα/dz caused by thespace charge: ##EQU2## with α=v_(r) /v_(z), wherein v_(r) and v_(z) arethe speed components of the particles in r- and z- direction. The valueof this function for any z can be determined with the aid of trajectorycalculations. The change of the beam position dR on the wafer planeowing to an angle change dα at the location z, dR/dα(Z), can likewise bedetermined by means of trajectory calculations, in that at a sufficientnumber of z-positions the calculation is interrupted, a slight anglechange Δα superimposed and then the calculation is continued until asfar as the wafer plane.

Equation (2) shows that in the environment of the cross-over it is notapplicable to assume a constant effective charge quantity for each ionsince in the cross-over point, i.e. where r(z)=0, the value for dα/dzwould become infinitely great. In actual fact, lens systems nevercomprise a sharp cross-over point, which means that equation (2) is notvalid for the environment of the cross-over point. If, however,sufficient negatively charged particles, e.g. electrons, are directed inthe region of the cross-over, then in this region the effect of thespace charge is equal to zero. It is therefore not necessary to takeinto consideration this region when calculating the integral of theequation (1).

By providing a sufficient number of electrons it is possible tocompensate the space charge in a region Z₁ <Z<Z₂. Diaphragms aredisposed at locations with the coordinates Z₁ and Z₂ before or after thecoordinate Z_(?) of the cross-over, which diaphragms are negativelybiased in order to ensure that no electrons can leave the region lyingbetween Z₁ and Z₂ around the cross-over. Since the operating sign ofdR/dα inverts after the cross-over and with respect to neutralising thespace charge between Z₁ and Z₂, equation (1) can be described asfollows: ##EQU3##

The coordinates z₁ and z₂ for the diaphragms are now selected so thatthe absolute amount of the two integrals of equation (3) is identical,in which case the deviation ΔR disappears.

This compensation of the space charge effect is, however, not limited tothe radially symmetrical space charge distribution.

In the case of an optional distribution of the mask holes the spacecharge field for a predetermined particle charge comprises both a radialand also an azimuthal component. The particle is therefore deflected bythe space charge radially by ΔR and azimuthally by the angle Δφ and thedistance RΔφ.

Equation (1) continues to apply for the radial change ΔR, and for theazimuthal deflection Δφ ##EQU4## analogously applies.

In so doing, dφ/dβ represents the azimuthal position change at thelocation of the wafer as a result of the beam direction change β,perpendicular to the main section (plane through the particle locationand optical axis), caused by the space charge and dβ/dz represents thechange of angle β per unit length caused by the space charge. Thevariables dR/dα and dφ/dβ and dα/dz and dβ/dz are associated with eachother in a convenient manner.

In the case of a rotationally symmetrical lens small angle changes Δαand Δβ in the main section and perpendicular thereto produce in thefirst order always identical deflections ΔR and RΔφ, i.e. always:##EQU5## for any values of Z.

Likewise dα/dz and dβ/dz are associated with each other in an extremelyconvenient manner, if again there is the prerequisite that the ratio ofthe radial distances of the two respective undisturbed ion paths remainconstant along the optical axis and there is the additional prerequisitethat the undisturbed ion paths constantly extend in the main section(which prerequisite is extremely well fulfilled in the case of the alsopointshaped ion sources). A similar charge distribution is then obtainedfor each plane perpendicular to the optical axis and thus for apredetermined ion path always a space charge field for the identicaldirection, which can be broken down at each location Z in the identicalmanner into a radial and azimuthal component. Thus, the following alsoapplies: ##EQU6## wherein the constant C does not depend upon Z butrather is determined only by the form of the charge distribution i.e. bythe distribution of the holes in the mask.

From equation (5) and (6) together with equation (1) and (4) it isevident that both for ΔR and also for Δφ a compensation of the spacecharge effects between suitably selected limits Z_(M), Z₁ and Z₂, Z_(w)occurs before and after the cross-over. Owing to equation (5) and (6)this compensating for ΔR and Δφ occurs at the same diaphragm positionsZ₁, Z₂. An extremely slight deviation from this simultaneouscompensation is merely caused by the astigmatism of the lenses used,which leads to slight deviations with respect to equation (5).

It is possible to achieve a precise match of the absolute amounts of theintegrals in equation (3) and thus an exact compensation of the spacecharge effect only for a predetermined value of R, i.e. for apredetermined desired impact location of a particle beam passing throughone mask hole onto the wafer plane. As it is not possible to setdifferent values for Z₁ and/or Z₂ for different values of R, thelocation of the beam impinging on the wafer changes for all not fullycompensated values of R with the current intensities used for theimaging. This change of position of the beam on the wafer dependent uponR means an additional distortion dependent upon the current. This can beexplained by the changed paths of the ions in the lenses caused by thespace charge, thus changing the effect of the lenses. This results inthe compensation of the spherical lens fields no long matching thecurrentless case.

In a further design of the embodiment according to the invention thecompensation of the spherical lens fields can be reproduced by changingin dependence upon the current the refractive powers of the lenses.

Calculations for an ion projection system (Austrian patent applicationA47/94 of the same applicant and identical date) have shown that byslightly changing the lens voltages the values of the distortion of theimage can be made equal to the same values as in the theoretical"currentless" case (without space charging).

A compensation of the global space charge according to the methodembodying the invention can be carried out even for the case that thecross-over is located within a lens and thus it is not possible in thisregion to supply in a controlled manner oppositely charged particles.Equation (2) then does not apply in the region of the cross-over anddα/dz must be determined in this region in a different way, e.g. bysimulation calculation. However, as long as the neutralising limits Z₁and Z₂ can be selected in such a manner that the two absolute amountsare identical in equation (3), according to equation (3) no change ofthe beam position at the wafer occurs as a result of the space charge.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail with reference to the followingdrawings.

FIG. 1 shows a schematic sectional view of a projection system forcharged particles between a mask and a wafer.

FIGS. 2 to 5 illustrate graphs of various functions for the region lyingbetween the mask and wafer in the z-direction (propagation direction ofthe beam as ordinate axis) and

FIGS. 6a and 6b illustrate the calculated progression of ion paths inthe environment of the cross-over.

DETAILIED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the propagation direction of the particle beam isdesignated z, the mask M and the wafer W. Z_(M) is the coordinate of themask plane and Z_(w) the coordinate of the wafer plane. The designation1 describes the progression of a beam passing through an hole 2 in themask M through two lenses L1 and L2 of the optical column. Thedesignation C describes the cross-over, i.e. the point the beam crossesthe ionic-optical (z-) axis. A device 3, which serves to introduce thecompensation electrons into the beam path, is located in the region ofthe cross-over C.

The numerals 4 and 4' or 5 and 5' describe diaphragms this side and theother side of the cross-over C, which lie on potentials V4, V4' or V5and V5'. The diaphragm holes 40, 41 or 50, 51 are to be selected of sucha size that all beams which carry image information resulting from thedifferent holes (transparent portions) in the mask M can passunhindered. The potentials V4 to V5' are selected such that theelectrons provided by the device 3 cannot escape from the region betweenthe diaphragms 4' and 5 or 4 and 5'. R is the distance in which the beampassing through the hole 2 impacts onto the wafer plane after beingdeflected by the lens system L1 and L2. The illustration is based on theassumption that the space charge effect was fully compensated, i.e. ΔRdisappears.

The graphs as shown in FIGS. 2 to 5 are based on an ion projector whichuses as lenses three-electrode grating lenses, wherein high energy ofapproximately 100 keV occurs at the wafer. FIG. 2 illustrates thefunction dR/dα which reduces between the mask and the cross-over C,wherein the cross-over is located approximately 1.3 m after the mask.After the cross-over C the function changes its operational sign and inthe wafer plane (3 m away from the mask M) becomes zero.

FIG. 3 illustrates the function dα/dz. In the region between thediaphragms whose coordinates are Z₁ and Z₂, the value of this functionis set to zero. The grating of the three electrode grating lenses islocated at the location Z_(G).

FIG. 4 illustrates the graph of the product (dR/dα)•(dα/dz).

Finally, FIG. 5 illustrates the integral for ΔR.

It is evident that after the cross-over C the effect of the space chargereduces, i.e. a partial compensation of the space charge effect occursirrespective of oppositely charged particles being provided. Byproviding a sufficient quantity of oppositely charged particlesprecisely in the region between Z₁ and Z₂, the effect of the spacecharge on the beam position becomes exactly equal to 0 at the locationof the wafer (where z=3m).

FIGS. 6a and 6b are based on an ion projector comprising a so-called twoelectrode grating lens, wherein the grating is formed by the mask (seeAustrian patent application A47/94 of the same applicant and identicaldate). FIG. 6a shows calculated ion paths in the environment of thecross-over with the neutralising limits Z₁, Z₂. If diaphragms areprovided at these locations, between which electrons are directed insufficient numbers, then ΔR in the equation (3) becomes zero. FIG. 6brepresents a greatly enlarged portion of FIG. 6a, from which it isevident that beams having different radii cross at the mask at differentlocations on the optical axis. FIG. 6b illustrates the regionapproximately 7 mm long, whereas FIG. 6a shows that the distance of thefirst diaphragm from the cross-over is greater than 50 mm. Thus, theassumption leading to equation (3) is sufficiently well fulfilled forthe ions entering in the neutralising region at Z_(t).

What is claimed is:
 1. A projection system for charged particles havinga mask which is disposed in a beam path of the charged particles andwhich comprises transparent portions including holes, which are disposedin a particularly asymmetrical manner with respect to an optical axisand which are imaged onto a wafer using lenses disposed in a beam pathof the charged particles, wherein the beam of the charged particlesincludes at least one crossover between the mask and the wafer forcrossing the optical axis, characterized in that in a defined regionbetween the mask and the wafer, charged particles which are oppositelycharged to the charge of the imaging particles are directed into thebeam path of the imaging particles, wherein limits defining the regionare selected such that an absolute amount of an integral effect of aspace charge on the particles imaging the mask structures is identicalin size before the defined region, as seen in the beam direction, as theabsolute amount of the integral effect of the space charge after thedefined region.
 2. The projection system according to claim 1,characterized in that the imaging particles are positively charged ionsand the oppositely charged particles are electrons.
 3. The projectionsystem according to claim 1, characterized in that the imaging particlesare negatively charged ions, more specifically negative H-ions and theoppositely charged particles are positively charged ions.
 4. Theprojection system according to claim 1, characterized in that adispensing device for the particles which are charged oppositely to theimaging particles is a LaB6 cathode which is designed as a LaB6 cylinderindirectly heated and encompassing the beam.
 5. The projection systemaccording to claim 1, characterized in that a dispensing device for theparticles which are charged oppositely to the imaging particles is anelectron cannon which emits in particular 1 keV electrons through a holeonto the inner wall of a metal cylindrical screen which encompasses thebeam, generating low energy secondary electrons which neutralize thebeam.
 6. The projection system according to claim 1, characterized inthat the imaging particles are electrons and the oppositely chargedparticles are positively charged ions.
 7. The projection systemaccording to claim 1, characterized in that the particles chargedoppositely to the imaging particles are directed to the beam path of theimaging particles in a region around the cross-over.
 8. The projectionsystem according to claim 7, characterized in that the size of theregion around the at least one cross-over is dimensioned such thatoutside this region for all paths of imaging particles, the ratio of theradial distances of the two respective paths remains constant from theoptical axis along the optical axis within previously fixed errorlimits.
 9. The projection system according to claim 1, characterized inthat in order to minimize a residual effect of the space charge therefractive powers of the lenses can be readjusted in dependence upon thetotal current intensity.
 10. The projection system according to claim 1,characterized in that in order to define the region in whichneutralizing particles are directed at least one diaphragm connected toa voltage source is disposed respectively on each side of the region andthe hole of the at least one diaphragm allows to pass through the beamof rays containing the image information originating from thetransparent portions in the mask.
 11. The projection system according toclaim 1, characterized in that a plurality of diaphragms are disposed onboth sides of the at least one cross-over and corresponding potentialsof the plurality of diaphragms can be adjusted individually, whereby thedefined region for provision of the charged particles can be varied. 12.The projection system according to claim 3, further comprising aplurality of diaphragms characterized in that the hole in at least oneof the diaphragms which is closer to the at least one cross-over issmaller than the hole in another one of the plurality of diaphragm lyingfurther away from the cross-over.